Macbook Pro unresponsive when unplugged

I am new to posting on this forum but I really hope that someone out there can help me out. I have a 13" MacBook Pro, 2.53 GHz Core 2 Duo, 4GB 1067 MHz DDR3, running OSX 10.6.8. My problem is that when I unplug the laptop form the charger and run it on the battery, the computer becomes unresponsive to any commands at all. It basically really slows down to a crawl. Even when rebooting it will take 5-10 minutes or more to do so. The odd thing is, when it gets plugged back in while any of this is going on, within a minute or two it works perfectly. It has been in a mac authorized repair shop for 7 weeks and has had a new logic board and a new hard drive replaced. The battery was tested and charges fine, holds a charge when unplugged, and the system profile shows the battery at Normal. Recently, many knowledgeable mac enthusiasts gave me tip that might help. Here is what I did: Ran an SMC reboot (no success), rebooted the computer under safe mode then unplugged it to test (no success), and ran a hardware test on the install disk (showed hardware ok)
I am at a loss as to what to do next. It has become impossible to use this laptop for my classes when it is unplugged. I am about ready to just go buy a new one, and call this one a goner.
I am hoping someone reading this can help with any other ideas on its problem. Help? Anyone?

If you done all that you say, including a SMC reset and the machine is acting the way it is, I would take it on back to Apple and have them trouble shoot it. Since they swapped out the logic board and hard drive it may still be under their repair warranty. Was this an Apple store or an authorized Apple dealer who did the work? It took them 7 weeks? Doesn't sound normal. Anyway, take it on back to them and insist they get it working right. But be polite....

Sorry for the late reply but I've been so frustrated over this. Apparently no one else can figure it out either. I took it back to the repair place that worked on it before and after running more tests, they found nothing wrong, not even the battery. It did do the slowdown for them though, but they couldn't figure it out. I then took it to the Apple store and they don't know either, but we did try something new that helped a little; we turned off all of the energy saver stuff on the battery settings. This does help somewhat, but it is not where it was before. So I'm still frustrated that no one is able to help me out. So it looks like I'm out of luck.....